1. Technical Field
The present invention relates to a prism sheet, and more particularly to a prism sheet in which each individual prism constituting a prism sheet has a concave pentagonal structure, thereby improving luminance and expanding the viewing angle thereof.
2. Background Art
A Liquid crystal display (hereinafter, referred to as a “LCD”) is one of image display devices, and has quickly replaced other display devices such as a cathode ray tube (CRT) due to various advantages, for example, its thin thickness and low electric power consumption, as compared with other display devices. In contrasted with the CRT, the LCD does not spontaneously emit a light ray and thus needs a back light unit, in addition to a liquid crystal window.
Conventional backlight units are categorized generally into an underneath type, an edge type and a taper type. The underneath type comprises at least two lamps arranged in parallel at the lower part of a light guiding plate (will be hereafter described), and the edge type has two lamps, but which are provided at both sides of the light guiding plate respectively. In the taper type, a single lamp is provided at one side of the light guiding plate, the lower part of which is slanted. Accordingly, the energy consumption increases in the order of the taper type, the edge type and the underneath type.
FIG. 1 is a perspective exploded view of a conventional taper-type backlight unit. FIG. 2 illustrates a perspective view of the construction of a conventional prism sheet. As shown in FIGS. 1 and 2, a reflective plate 4, a light guiding plate 2, a diffusion plate 5, a lower prism sheet 6, an upper prism sheet 7, and a protection plate 8 are stacked in sequence above a mold frame 9. Although not illustrated, a liquid crystal panel is placed above and spaced apart from the protection plate 8. A lamp 1 is disposed at one side of the light guiding plate 2 and enclosed with a lamp reflector 3.
On the other hand, as shown in FIG. 2, the conventional backlight unit comprises the upper prism sheet 7 and the lower prism sheet 6, which are stacked one above the other. The lower prism sheet 6 consists of a plurality of elongated projections 6a, which are arranged in parallel. Each of elongated projection runs in the same direction and has a transversal cross-section of isosceles triangle, thereby forming a prism. Similarly, the upper prism sheet 7 has a plurality of elongated projections 7a thereon, which have the same configuration as the lower projections 6a, with exception that the upper and lower projections 6a, 7a are arranged perpendicularly to each other.
In the conventional backlight unit having the above-described construction, a light ray emitted from the lamp 1 passes the light guiding plate 2 directly or after reflected on the reflector 3, and then, via the diffusion plate 5 and the prism sheets 6 and 7, enters the liquid crystal panel, where resultantly a desired image is formed. At this time, the light ray emitted from the lamp 1 enters the light guiding plate without leaking outside by means of the reflector 5, and is uniformly scattered therein. The scattered light ray is made more uniform while passing the diffusion plate 5. The light ray emitting from the diffusion plate 5 is refracted at a specific angle when pass the prism sheets 6 and 7, and finally enters the liquid crystal panel. In particular, the light ray is uniformly aligned at the orientation of 90 or 180 degrees and amplified while passing the orthogonal projections 6a, 7a of the prism sheets 6, 7. The conventional backlight unit described above is exemplified by Koran Patent Publication No. 58166 (published on Jul. 5, 2001), and other type of backlight unit is disclosed in Korean Patent Publication No. 55377 (published on Jul. 4, 2003).
However, the conventional prism sheet is structured such that each individual prism has a cross-section of isosceles triangle, in which only two oblique sides (legs of the isosceles triangle) can make contribution to the increase in luminance, thereby resulting in a limited efficiency of increasing the luminance.